Conventionally, a so-called disk filter has been employed to filter water for various uses such as irrigation.
The disk filter has a configuration in which a plurality of plate-shaped annular filter disks are housed in a filter case such that the filter disks are fitted around the outer periphery of a center shaft and are vertically stacked on one another.
The front surface and the rear surface of each filter disk are provided with grooves extending from the outer end (outer periphery) toward the inner end (inner periphery) in the radial direction. With such grooves, water channels (water introduction paths) are defined between the groove on the front surface of the first filter disk and the rear surface of the second filter disk when the first filter disk and the second filter disk are stacking on each other, for example.
In addition, the filter case includes an inlet from which water supplied from the water source enters, and an outlet from which filtered water is output to a supply destination (downstream side).
In the disk filter, at the time when water having entered the filter case from the inlet flows into the channels defined by the grooves of the filter disks, foreign matters in the water are captured at the peripheral surfaces or channels of the filter disks, and thus the water is filtered.
Such a disk filter is disclosed in PTL 1 for example.
However, the shape of the groove of the disk filter disclosed in PTL 1 is a straight shape. Therefore, the aperture ratio on the outer periphery side of the filter disk is smaller than the aperture ratio on the inner periphery side of the filter disk. Here, the aperture ratio is the ratio of the total area of all opening parts of the grooves on the outer peripheral surface or the inner periphery surface of the filter disk, with respect to the virtual total area of the outer peripheral surface or the inner periphery surface provided with no groove.
With such a configuration, when the water supplied to the disk filter from the water source side flows into the channel defined by the groove from the outer periphery side of the filter disk (that is, the outer peripheral surface of the filter disk is used as a surface for capturing foreign matter), the water cannot readily flow into the channel.
Accordingly, the disk filter disclosed in PTL 1 has a problem that the hydraulic pressure has to be increased by using a high pressure pump on the water source side to appropriately perform filtration.
Examples of the method for increasing the aperture ratio on the outer periphery side of the filter disk include, for example, a method in which the width of the groove is increased from the inner periphery side toward the outer periphery side of the filter disk, and a method in which the depth of the groove is increased from the inner circumference side toward the outer periphery side.
However, in the former method, the width of the groove gradually increases from the inner circumference side toward the outer periphery side. As a result, with the former method, large foreign matters easily enter the channel, degrading the filtration performance.
In addition, the latter method has a problem that a hole is undesirably defined in the filter disk when the grooves on the front surface and the rear surface are defined at a position where the grooves overlap each other in the thickness direction of the filter disk.
On the other hand, PTL 2 discloses a metal filter in which a curved groove is provided to the surface of a metal plate member. With such a filter having the above-mentioned configuration, the aperture ratio on the outer periphery side can be increased while avoiding the above-mentioned two problem defect.
However, with the curved groove disclosed in PTL 2, it is difficult to limit pressure drop (in other words, the pipe resistance of a channel defined by a groove), in comparison with the case of a straight groove.
In particular, increase in pressure drop cannot be avoided in the case where the curvature change of a groove is abrupt as with groove 8c disclosed in FIG. 10 and groove 8e disclosed in FIG. 11 in PTL 2, and in the case where the groove is unnecessarily long as in FIG. 12 of PTL 2.
Therefore, as with the case of the disk filter having the straight groove, the filter disclosed in PTL 2 also requires a high pressure pump to appropriately perform filtration when the resulting pressure drop is taken into account. As described, even when the aperture ratio is increased with the above-mentioned grooves, the pressure drop may not have been decreased.